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><A
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>11.3. Loop Control</H1
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><I
><P
><I
>Tournez cent tours, tournez mille tours,</I
></P
><P
><I
>Tournez souvent et tournez toujours . . .</I
></P
><P
><I
>--Verlaine, <SPAN
CLASS="QUOTE"
>"Chevaux de bois"</SPAN
></I
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><P
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="BRKCONT1"
></A
>Commands affecting loop behavior</B
></P
><DL
><DT
><B
CLASS="COMMAND"
>break</B
>, <B
CLASS="COMMAND"
>continue</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>break</B
> and <B
CLASS="COMMAND"
>continue</B
>
	      loop control commands
		<A
NAME="AEN6981"
HREF="#FTN.AEN6981"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>
	      correspond exactly to their counterparts in other
	      programming languages. The <B
CLASS="COMMAND"
>break</B
>
	      command terminates the loop (<EM
>breaks</EM
>
	      out of it), while <B
CLASS="COMMAND"
>continue</B
> causes a jump
	      to the next <A
HREF="loops1.html#ITERATIONREF"
>iteration</A
>
	      of the loop, skipping all the remaining commands in that
	      particular loop cycle.</P
><DIV
CLASS="EXAMPLE"
><A
NAME="EX28"
></A
><P
><B
>Example 11-21. Effects of <I
CLASS="FIRSTTERM"
>break</I
> and
		<B
CLASS="COMMAND"
>continue</B
> in a loop</B
></P
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><TD
><FONT
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><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash

LIMIT=19  # Upper limit

echo
echo "Printing Numbers 1 through 20 (but not 3 and 11)."

a=0

while [ $a -le "$LIMIT" ]
do
 a=$(($a+1))

 if [ "$a" -eq 3 ] || [ "$a" -eq 11 ]  # Excludes 3 and 11.
 then
   continue      # Skip rest of this particular loop iteration.
 fi

 echo -n "$a "   # This will not execute for 3 and 11.
done 

# Exercise:
# Why does the loop print up to 20?

echo; echo

echo Printing Numbers 1 through 20, but something happens after 2.

##################################################################

# Same loop, but substituting 'break' for 'continue'.

a=0

while [ "$a" -le "$LIMIT" ]
do
 a=$(($a+1))

 if [ "$a" -gt 2 ]
 then
   break  # Skip entire rest of loop.
 fi

 echo -n "$a "
done

echo; echo; echo

exit 0</PRE
></FONT
></TD
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></DIV
><P
><A
NAME="BREAKPARAM"
></A
></P
><P
>The <B
CLASS="COMMAND"
>break</B
> command may optionally take a
	      parameter. A plain <B
CLASS="COMMAND"
>break</B
> terminates
	      only the innermost loop in which it is embedded,
	      but a <B
CLASS="COMMAND"
>break N</B
> breaks out of
	      <TT
CLASS="PARAMETER"
><I
>N</I
></TT
> levels of loop.</P
><DIV
CLASS="EXAMPLE"
><A
NAME="BREAKLEVELS"
></A
><P
><B
>Example 11-22. Breaking out of multiple loop levels</B
></P
><TABLE
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><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# break-levels.sh: Breaking out of loops.

# "break N" breaks out of N level loops.

for outerloop in 1 2 3 4 5
do
  echo -n "Group $outerloop:   "

  # --------------------------------------------------------
  for innerloop in 1 2 3 4 5
  do
    echo -n "$innerloop "

    if [ "$innerloop" -eq 3 ]
    then
      break  # Try   break 2   to see what happens.
             # ("Breaks" out of both inner and outer loops.)
    fi
  done
  # --------------------------------------------------------

  echo
done  

echo

exit 0</PRE
></FONT
></TD
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></TABLE
></DIV
><P
>The <B
CLASS="COMMAND"
>continue</B
> command, similar to
	      <B
CLASS="COMMAND"
>break</B
>, optionally takes a parameter. A
	      plain <B
CLASS="COMMAND"
>continue</B
> cuts short the
	      current iteration within its loop and begins the next.
	      A <B
CLASS="COMMAND"
>continue N</B
> terminates all remaining
	      iterations at its loop level and continues with the
	      next iteration at the loop, <TT
CLASS="OPTION"
>N</TT
> levels
	      above.</P
><DIV
CLASS="EXAMPLE"
><A
NAME="CONTINUELEVELS"
></A
><P
><B
>Example 11-23. Continuing at a higher loop level</B
></P
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><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# The "continue N" command, continuing at the Nth level loop.

for outer in I II III IV V           # outer loop
do
  echo; echo -n "Group $outer: "

  # --------------------------------------------------------------------
  for inner in 1 2 3 4 5 6 7 8 9 10  # inner loop
  do

    if [[ "$inner" -eq 7 &#38;&#38; "$outer" = "III" ]]
    then
      continue 2  # Continue at loop on 2nd level, that is "outer loop".
                  # Replace above line with a simple "continue"
                  # to see normal loop behavior.
    fi  

    echo -n "$inner "  # 7 8 9 10 will not echo on "Group III."
  done  
  # --------------------------------------------------------------------

done

echo; echo

# Exercise:
# Come up with a meaningful use for "continue N" in a script.

exit 0</PRE
></FONT
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></DIV
><DIV
CLASS="EXAMPLE"
><A
NAME="CONTINUENEX"
></A
><P
><B
>Example 11-24. Using <I
CLASS="FIRSTTERM"
>continue N</I
> in an actual task</B
></P
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><PRE
CLASS="PROGRAMLISTING"
># Albert Reiner gives an example of how to use "continue N":
# ---------------------------------------------------------

#  Suppose I have a large number of jobs that need to be run, with
#+ any data that is to be treated in files of a given name pattern
#+ in a directory. There are several machines that access
#+ this directory, and I want to distribute the work over these
#+ different boxen.
#  Then I usually nohup something like the following on every box:

while true
do
  for n in .iso.*
  do
    [ "$n" = ".iso.opts" ] &#38;&#38; continue
    beta=${n#.iso.}
    [ -r .Iso.$beta ] &#38;&#38; continue
    [ -r .lock.$beta ] &#38;&#38; sleep 10 &#38;&#38; continue
    lockfile -r0 .lock.$beta || continue
    echo -n "$beta: " `date`
    run-isotherm $beta
    date
    ls -alF .Iso.$beta
    [ -r .Iso.$beta ] &#38;&#38; rm -f .lock.$beta
    continue 2
  done
  break
done

exit 0

#  The details, in particular the sleep N, are particular to my
#+ application, but the general pattern is:

while true
do
  for job in {pattern}
  do
    {job already done or running} &#38;&#38; continue
    {mark job as running, do job, mark job as done}
    continue 2
  done
  break        # Or something like `sleep 600' to avoid termination.
done

#  This way the script will stop only when there are no more jobs to do
#+ (including jobs that were added during runtime). Through the use
#+ of appropriate lockfiles it can be run on several machines
#+ concurrently without duplication of calculations [which run a couple
#+ of hours in my case, so I really want to avoid this]. Also, as search
#+ always starts again from the beginning, one can encode priorities in
#+ the file names. Of course, one could also do this without `continue 2',
#+ but then one would have to actually check whether or not some job
#+ was done (so that we should immediately look for the next job) or not
#+ (in which case we terminate or sleep for a long time before checking
#+ for a new job).</PRE
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HSPACE="5"
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><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <B
CLASS="COMMAND"
>continue N</B
> construct is
	      difficult to understand and tricky to use in any meaningful
	      context. It is probably best avoided.</P
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><H3
CLASS="FOOTNOTES"
>Notes</H3
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><A
NAME="FTN.AEN6981"
HREF="loopcontrol.html#AEN6981"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>These are shell <A
HREF="internal.html#BUILTINREF"
>builtins</A
>,
		whereas other loop commands, such as <A
HREF="loops1.html#WHILELOOPREF"
>while</A
> and <A
HREF="testbranch.html#CASEESAC1"
>case</A
>, are <A
HREF="internal.html#KEYWORDREF"
>keywords</A
>.</P
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